Astron. Astrophys. 335, 1040-1048 (1998)

1. Introduction

There now exist numerous examples of collimated outflows and jets from young, pre-main sequence (PMS) stars. Many such phenomena have been imaged in the optical where they manifest themselves via shock-excited nebulous structures generally referred to as Herbig-Haro (HH) objects (Herbig 1951; Böhm et al. 1973) and Herbig-Haro jets (e.g. HH 46/47, Dopita, Evans & Schwartz, 1982; HH 34, Reipurth & Heathcote, 1992; HH 111, Reipurth, 1989). Also often present in outflows with HH objects are parabolic bow shock structures indicating the existence of a `working-surface' where the outflowing gas jet impacts on and interacts with either ambient molecular cloud material or earlier ejecta from the young star. A excellent example of an optical bow shock is found in the source HH 34 (Reipurth & Heathcote, 1992). In this object a large parabolic emission feature is located beyond, and along the axis of, a thin high-velocity HH jet originating close to the young star (cf. HST WFPC2 image in PRC95-24a). Some collimated outflows, however, cannot be studied in the optical since they remain deeply embedded in the dense molecular cores out of which the associated young stellar objects (YSOs) are forming. These objects are obscured by perhaps magnitudes of extinction. Some of these embedded sources have been imaged with highly sensitive, large-format near-IR (NIR) arrays instruments and it is becoming clearer that similar morphological structures as seen in the optical are present also in shock-excited NIR emission, specifically in the lines of molecular hydrogen (H₂). Good examples of such sources are HH 211 (McCaughrean, Rayner & Zinnecker 1994), L1634 (Hodapp & Ladd, 1995; Eislöffel, 1997) and L1448 (Davis et al., 1994a). In addition, many optically detected HH objects and jets have subsequently been studied in the NIR (e.g. HH 46/47, Eislöffel et al., 1994; HH 1/2, Davis et al., 1994b). In these objects shock-excited H₂ emission is often observed coincident with, or closeby, regions of optical shock-excited emission. Below, we presented the discovery of multiple NIR bow shocks structures and associated nebulosities around the PMS binary AFGL 961. Whether these objects are the NIR counterparts of optical shock-excited emission nebulae i.e. the NIR equivalent of optical HH objects, or are purely NIR emission nebulae is unclear due to overlying extinction. However, to date, such structures, especially associated with high-mass, high-luminosity PMS binaries, are unique and are suggestive of a high level of outflow activity in the source in the recent past.

The source AFGL 961 (Grasdalen, et al. 1983) is a well known and well studied high-mass, high-luminosity ( 7500, Castelaz, et al. 1985) binary PMS star system located in the outskirts of the Rosette Nebula (Lenzen, et al. 1984). The region surrounding AFGL 961 is optically obscured by the associated molecular cloud core, however, the two stellar components of the system are easily detected in the NIR. These two sources are both classified as early-B type PMS stars and are separated by or, at the distance of AFGL 961 (d1.6kpc, Turner, 1976), 9600 A.U. or 0.04pc. Both stars possess 2.166µm Br- emission and an associated bipolar CO molecular outflow (Lada & Gautier 1982) enamates from their location and extending preferentially to the north-east and south-west ¹. There exists an associated IRAS source, 06319+0415, centred on the binary with fluxes at 12 µm, 25 µm, 60 µm, 100 µm of 78 Jy, 375 Jy, 959 Jy and 995 Jy, respectively. NIR K-band images of the region presented by Hodapp (1994) show it to be morphologically complex with extensive nebulosity surrounding the central binary. Hodapp suggested that a small stellar PMS cluster may be present around AFGL 961 including the bright PMS star, designated AFGL 961 W, 30" to the west of the binary. Due to this previous designation, we herein refer to the eastern and western AFGL 961 binary components as AFGL 961a and AFGL 961b, respectively.

The two stars forming the AFGL 961 binary were included in a 2µm spectroscopic survey of young, high-mass, high-luminosity, pre-main sequence object we performed at the UKIRT telescope. During this survey we detected strong molecular hydrogen (H₂) emission to both the north and south of AFGL 961b. This discovery led us to a more focussed observing campaign. The results of this study are presented below. We show that AFGL 961 possess numerous shock-excited nebulous H₂ objects located along many different axes from the sources and lying up to 110" (0.85pc at 1.6kpc) distant. Several of these nebulous objects are similar in morphology to optical HH bow shocks. Below we discuss the discovery, nature and possible history of these multiple, non-axially symmetric H₂ bow shock structures.

© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998
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